The invention relates to maskless lithography, and relates in particular to maskless lithography using arrays of focusing elements.
U.S. Pat. No. 5,900,637 discloses a system for maskless lithography that involves the focusing of incident energy from a single energy source onto a substrate as focused beamlets. The arrangement includes micro-mechanical devices with actuatable shutters that turn the focused beams on and off in response to commands from a control computer. To write a pattern, the substrate is scanned under the array, while the individual beams are turned on and off as needed by means of the micro-mechanical shutters, one associated with each zone plate. These shutters are disclosed to be located either between the zone plate array and the substrate, or between the zone plate array and the source of radiation. The '637 patent also discloses a lithography system that includes an array of micro-mechanical, deflectable glancing-angle mirrors that may be used to turn individual focused beams on and off.
Such lithography systems, however, require that zone plate arrays be very precise and uniform without significant variation. Diffractive elements such as zone plates may be formed using step approximation, which requires aligned lithography and an etch for each level of stepping in forming the diffractive elements. Other diffractive elements may be formed by 3D patterning in which grayscale resist exposures are used, or using a low selectivity reactive ion-etching step. In certain applications, however, the use of individually created zones on zone plates may not be appropriate.
For example, a conventional process for fabricating UV and DUV zone plates typically involves defining the area where the diffractive optics would be placed in subsequent steps. Chromium (Cr) is used as the absorber material in this process, but a number of other materials may also be employed. The diffractive optics are then defined, and the resist patterns are then transferred into the fused silica substrate by means of reactive-ion-etching. This and other conventional fabrication procedures requiring multilevel alignment are difficult to generalize to create very large arrays with commercial high-throughput electron-beam-lithography systems. The chief problem arises from the fact that commercial mask makers (both electron-beam-lithography and laser based systems), the only systems with sufficient throughput to write large arrays, are not set-up for multi-level alignment.
There remains a need therefore, for an improved method of forming arrays of zone plates or other diffractive elements for a maskless lithography system.